Add contributing doc

Signed-off-by: Serhii Horodilov <sgorodil@gmail.com>

Author: shorodilov

.github/CONTRIBUTING.md

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+# Contributing to the Problem Sets Repository
+
+Welcome to the Training Repository! We welcome all contributions from
+the community. Please take a moment to review this guide before getting
+started.
+
+## Getting Started
+
+1. Fork the repository to your own GitHub account.
+2. Clone the repository to your local machine.
+3. Install project dependencies
+4. Create a new branch with a descriptive name for your feature or bug fix.
+
+## Making Changes
+
+1. Make changes to the code, documentation, or any other aspect of
+   the repository.
+2. Commit your changes with a clear and descriptive commit message.
+3. Push your changes to your forked repository.
+4. Create a pull request to the main repository, detailing your changes
+   and their impact.
+
+## Code Style
+
+We follow the [PEP 8](https://www.python.org/dev/peps/pep-0008/) code
+style guide. Please ensure your code adheres to these standards.
+
+## Testing
+
+Before submitting your pull request, please ensure that all tests pass.
+We use [pytest](https://docs.pytest.org) for testing.
+To run the tests, use the command:
+
+```shell
+pytest
+```
+
+Thank you for your contributions to the Training Repository!